This invention is related to flanged connection joints for valve bodies. More particularly the invention is related to a body-bonnet joint for a valve which has a removably mounted bonnet secured to the valve body by a flanged connection.
In the prior art, flange connections between the bonnet and body on a gate valve are quite common as are flanged connections on the other portions of a valve body when such are provided. These flanged connections generally include some geometrical feature on both flange portions which locates the flange surfaces in a concentric relation. Also, commonly the flanges include some type of seal between the flanges to achieve fluid-tight sealing. It is well known in the art to provide an O-ring mounted in a groove in one or both of the flange members to seal between both flanges. A common problem in valve construction is providing joints which will withstand high temperature conditions which are encountered in some services and for emergency conditions such as a fire. When an elastomeric O-ring is mounted between the flange surfaces and it is exposed to high temperature conditions, the O-ring is usually blown out of the groove once the temperature becomes sufficient to permit the bolting around the flanged joint to relax thereby permitting separation of the flanges and providing an escape route for the O-ring. It is quite common for high temperature seal ring constructions to have a metallic O-ring or other substantially rigid seal material because of their resistance to high temperatures. However, these materials are not very resilient, thus, only a minor relaxation of the bolting is necessary to separate the sealing surfaces and create a leak. It is to be noted that most flange joints for the bonnet connection of a valve have some type of centralizing or locating structure to position the flanges with respect to each other and thus locate the stem passageway aperture in the bonnet relative to the valve member. Also, these typical flange connections have the locating structure and the flange joint seal on the interior of the bolt circle. By locating the flange seal inside the bolt circle, sealing around the bolts or fasteners is not necessary. Another feature common to the prior art is that compression of the flange seal member or gasket is provided only by tension of the bolting and thus relaxation of the bolt tension directly reduces the sealing ability of the flange joint. The typical construction is not suitable for valve installations in a high temperature environment because the bolts will relax or the valve body and bonnet structure will change shape and a leak is created.